The invention relates to automatic blood pressure monitors, and more particularly relates to the inflation pump used to inflate the inflation cuff. In its most immediate sense, the invention relates to circuitry which is used to control a DC motor that powers the inflation pump, and which is capable of properly inflating even the small inflation cuffs used for neonates.
In an automatic blood pressure monitor, an inflation cuff is slipped onto e.g. the patient's upper arm and inflated to a pressure which exceeds the patient's systolic pressure. This collapses the main artery in the patient's arm and cuts off blood flow to the lower arm. Then, the inflation cuff is slowly deflated while the pressure inside the cuff is monitored using a pressure transducer. The patient's systolic and diastolic pressures can then be determined by monitoring the output of the pressure transducer (which responds to variations in cuff pressure caused by the patient's pulse) and correlating that output with the pressure within the cuff.
While the cuff must be inflated to an inflation pressure which exceeds the systolic pressure, the excess should not be great because this unnecessarily prolongs the measurement process and also unnecessarily stresses the patient's arm. To accurately control the inflation process and cease cuff inflation at an appropriate pressure, it is advantageous to have the pump deliver air at more than one rate. This is because automatic blood pressure monitors are used with cuffs that vary greatly in size; a large cuff may be large enough to encompass the human thigh, while a neonatal cuff may be so small as to just fit over an adult's little finger. The rate of inflation of a neonatal cuff should be only a small fraction--perhaps 1/5--of the rate of inflation of a large cuff.
In the past, as is disclosed in commonly-owned U.S. Pat. No. 5,240,008, a pump powered by a DC motor has been used to inflate the cuff, and the rate of air delivery has been adjusted by using valves to vary the supply of intake air to the pump. Such an adjustment scheme requires the use of relatively expensive electromechanical components. It would be advantageous to eliminate the use of such components if possible and to vary the rate of air delivery by appropriately controlling the speed of the pump motor.
It would be advantageous to provide an automatic blood pressure monitor which varied the rate of air delivery from the pump by controlling the speed of the DC pump motor using a multi-speed control circuit, which control circuit would properly regulate the air output of the pump at a high volume rate and at a low volume rate which is small as compared with the high volume rate.
One object of the invention is to provide an automatic blood pressure monitor which will pump air into the cuff both at a high volume rate and at a low volume rate which is small as compared with the high volume rate, and which operates by varying the speed of a DC motor used to drive the pump.
Another object is, in general, to improve on known automatic blood pressure monitors of this general type.
In accordance with the invention, the air pump is of a type (e.g. a triple-diaphragm type) which produces an acceptably low pressure ripple at low speeds of the motor shaft. In further accordance with the invention, use is made of a well-known characteristic of a DC motor, namely that the motor winding of a DC motor generates an EMF (a voltage) which is directly proportional to the speed of the motor. In accordance with the invention, this EMF is determined and a feedback loop is used to maintain this EMF at a constant value to maintain the motor speed at a highly constant rate in spite of variations in load. In further accordance with the invention, this feedback loop is operated in one of two modes. In the first mode, a high volume rate mode, the DC pump motor is operated at a high speed. In the second mode, a low volume rate mode, the DC pump motor is operated at a low speed which is small as compared with the high speed.
Advantageously, and in accordance with the preferred embodiment, there is a second feedback loop which is used to control motor current. This prevents excessive current consumption when the motor is started.
Further advantageously, and in further accordance with the preferred embodiment of the invention, the high speed mode causes the motor to be operated at its maximum speed.
Further advantageously, and in accordance with the preferred embodiment, when the motor is turned off, it is first disconnected from power and later dynamically braked by shunting through a low resistance; this stops the motor sooner and thus reduces the cuff pressure overshoot from that which existed when the pump was switched off.